1. Field of the Invention
This invention relates generally to nickel hydroxide containing electrodes for use in a rechargeable cell and, more particularly, to positive nickel hydroxide electrodes having enhanced nickel hydroxide utilization for use in nickel metal hydride cells. Specifically, the present invention relates to nickel hydroxide positive electrodes for use in nickel metal hydride electrochemical cells particularly of the AB5-type.
2. Description of the Prior Art
Sealed or semi-sealed nickel metal hydride alkaline cells employ hydrogen storage alloy materials as the electrochemically active material for the negative electrode. These alloy materials are believed to possess catalytically active sites for disassociating hydrogen molecules to increase the rate at which hydrogen absorption and desorption can be accomplished. Cells made with these hydrogen storage alloy materials forming the basis of the negative electrode are typically matched against nickel positive electrodes with alkaline electrolyte in a sealed configuration operating in some respects analogously to the nickel-cadmium cell.
The metal hydride electrodes may be of the pasted or compressed powdered type as well as a sintered type. Sintered nickel containing electrodes, and in particular positive nickel hydroxide electrodes, are typically formed by sintering nickel powder at elevated temperatures and then impregnating the porous sintered structure with the active nickel hydroxide material as well as other materials. Pressed or extruded paste nickel hydroxide electrodes are produced by physically depositing and then pressing the active nickel hydroxide material and other active materials into contact with an appropriate current collector substrate. In each of these types of electrodes typically used in rechargeable electrochemical cells, the principal active material comprises nickel hydroxide as a main constituent. In order to adhere the active material to the current collector substrate in the pasted type of electrode, additional materials such as organic binders, i.e., polyacrylate and polytetrafluoroethylene, may be utilized, as well as conductive diluents such as graphite powder or fibers or nickel powder.
There are a wide variety of prior art references that discuss or address nickel metal hydride cells in general as well as regarding particular aspects thereof. U.S. Pat. No. 4,820,481, for example, illustrates a method for the continuous fabrication of negative electrodes for use in such cells. U.S. Pat. No, 2,724,733 discloses a sintered electrode structure for a nickel cadmium cell having nickel hydroxide as the active material for the sintered positive electrode. U.S. Pat. No. 3,826,684 also discloses a nickel cadmium cell having a positive nickel hydroxide electrode containing additives to enhance storage capacity. These and other general references in the prior art are all cognizant of the use of nickel hydroxide as a positive electrode in nickel cadmium cells as well as nickel metal hydride cells. Moreover, additives to the nickel hydroxide to enhance cell performance and capacity are known.
U.S. patent application Ser. No. 07/714,963 filed Jun. 13, 1991, now abandoned, and assigned to the same assignee as the subject application, discloses the use of cobalt as an additive to enhance the performance of positive nickel hydroxide electrodes, the contents of which are specifically incorporated herein by reference. This application relates to both nickel cadmium electrochemical cells as well as to nickel metal hydride electrochemical cells. Nonetheless, increasing emphasis is being placed on the nickel metal hydride electrochemical cells as replacements for nickel cadmium cells due to environmental concerns relating to cadmium. Thus, the use of cadmium in any capacity, including as an additive, in electrochemical cells is being discouraged. Consequently, additive schemes which are applicable to nickel cadmium cells may or may not be equally applicable to nickel metal hydride cells due to the difference in the operation of such cells as further explained below. Consequently, there is still a need for enhancing the performance and capacity factors of nickel metal hydride electrochemical cells, particularly of the AB5-type.